Electron Appetites

Chm 1311 Lecture for 21 June 2000

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Polar Covalent Bonds	If there are bonds which are strictly ionic, such as [Na⁺] [Cl^-], and those which are just as strictly covalent, such as Cl₂, where neither Cl has the advantage, there are most likely bonds where electrons are neither shared equally nor transfer completely. These are called polar covalent or sometimes just polar bonds. Carbon dioxide, CO₂, is a molecule with such polar bonds. The electrons in those O=C=O double bonds are unequally shared. Not surprisingly, oxygen, with its much greater electron affinity, gets more than its share, and each C=O bond is polarized...but not ionized, that is, the molecule is not [O^2-] [C⁴⁺] [O^2-]. But while both bonds are polar, the CO₂ molecule is not polar! The explanation behind this seeming paradox is that O=C=O is linear, and the dipole vector stretching left from C to the first O is vectorially cancelled by an identical one stretching right O C O Hence the symmetry of carbon dioxide prevents the polar bonds from becoming a polar molecule.
Polar Molecules	In contrast, the water molecule, H₂O, is not linear but rather bent. It's Lewis structure could be drawn .. H : O : H " since Lewis structures aren't meant to show the 3d structure of the molecule. (After all, the structures are all drawn in a plane; do we imagine that therefore all molecules are flat? Not a bit of it.) But we'd do greater justice to water if we made its Lewis structure .. H : O : " H
	Since O has a much higher electron affinity than H, we'll expect the O-H bonds to be polar, but in water, the dipolar vectors do not cancel vectorially. They add (vectorially) to a non-zero sum. (The magnitude depends on both the polarity and the exact angle of those vectors, but we'll not belabor that here.) HO H While it's true that the water molecule has symmetry (symmetry elements, like its rotation axis midway between the hydrogens and the two mirror planes that contain it, are a fascinating feature of molecules that we'll visit in later courses), it doesn't have CO₂'s inversion symmetry that cancels CO₂'s dipoles. So water is indeed polar.
Polar Water	As we've mentioned in class many times, water's polarity gives it many of the life-critical properties we see in it. Its polarity makes for strong hydrogen bonding which keeps water liquid even while the heavier H₂S is a gas! Being creatures of liquid water, we're unknowingly beholden to hydrogen bonds. Likewise its polarity makes it a wonderful solvent for ionic molecules since it reorients itself near the ionic charge to create a polar cage to trap the ion. Better still, the water molecules between the ions reorient to nearly cancel the electrostatic field between cations and anions, keeping them from clustering. But is it only oxygen's rapacious electron affinity which turns this trick? No. It is necessary for oxygen to hold onto its own electrons as well as make a successful bid for electrons of others which ensures the polarity of many of its bond combinations. So if we were to seek a Holy Grail of polarity or its opposite, we ought to look at both electron affinity and ionization potential.
Electro- Negativity Linus Pauling MS Encarta 97	Linus Pauling, the only person to win unshared Nobel Prizes in two fields (Chemistry and Peace), revolutionized Chemistry and kick started molecular biology by applying principles of quantum mechanics in common sense ways to molecules. One of his many innovations was electronegativity which he took as a scaled bond energy left over after shared bonding was estimated. While electronegativity has been more carefully defined subsequently, that tension between "how much do I want what I got" and "how much do I want what YOU got" is a telling feature of electronegativity. Of course, it's really the negative of electron affinity that's used, since a very negative electron affinity means that the potential of the atom lowers if you give it another electron. But the sign change in nitpicking; the important feature is that small IP and small EA give a small electronegativity, designated as c, the Greek letter "chi". Small c means an atom relatively uninterested in holding electrons while a large c betokens one very hungry for electrons.
	The largest electronegativity belongs to the fluorine atom with c=4.1 And the smallest to cesium (and francium) with c=0.9. That makes the difference between the largest and smallest Dc of 3.2 for the CsF (highly ionic) bond. In fact, bonds with Dc >2 are considered highly ionic. Those with Dc <1 are quite covalent. Leaving the majority of molecules in the range 1 to 2 as polar covalent.
Sample Electro- negativities	H:2.1 Li:1.0 Be:1.5 B:2.0 C:2.5 N:3.1 O:3.5 F:4.1 Na:1.0 Mg:1.3 Al:1.5 Si:1.8 P:2.1 S:2.4 Cl:2.9 K:0.9 Ca:1.1 Ga:1.8 Ge:2.0 As:2.2 Se:2.5 Br:2.8 Rb:0.9 Sr:1.0 In:1.5 Sn:1.7 Sb:1.8 Te:2.0 I:2.2 Cs:0.9 Ba:0.9 Tl:1.5 Pb:1.6 Bi:1.7 Po:1.8 At:2.0 One of the dull combinations is C-H with a Dc only 0.4, clearly not ionic. No wonder that hydrocarbons, C_nH_2n+2, don't want to mix with water. Water dissolves polar creatures rather well, but whatever the geometry of the hydrocarbon, no symmetry or lack thereof can make up for the fact that each and every bond is hopelessly covalent.
Organic Chemistry	Brady sneaks organic chemistry into many chapters hoping to make it palatable by accretion, no doubt. And while many organic molecules, like that for spearmint oil (at right), are unquestionably fascinating, the subject is rich enough to deserve its own chapter (#21) and more rather than being trotted out piecemeal. Still, organic compounds are all around us, and, with the exception of water, constitute the majority of each of us! So getting familiar with some of them early on appears to be Brady's goal. So it probably wouldn't hurt to pop off to a few lectures on organic chemistry to get your feet wet. If you find your own appetite whetted, there are countless webpages devoted to organic chemistry tutorials!